PROJECT SUMMARY Young adult cancer survivors (YAs) are an important underserved group at risk for significant psychological distress. There are approximately 70,000 new diagnoses of cancer annually in YAs (ages 18-39), and currently nearly 2 million people in the United States are living with or have survived being diagnosed with cancer as a YA. Five-year survival rates of YAs are high (>80%) and YAs have approximately 35 to 59 years of life expectancy remaining, underscoring the importance of post-treatment survivorship care. YAs face unique challenges given the physical, cognitive, and psychosocial developmental milestones disrupted as a result of cancer and are at greater risk of psychological distress compared to older adults (ages ?40) with cancer. Accordingly, YAs can benefit from targeted, supportive care interventions to decrease distress and enhance well-being as they navigate post-treatment survivorship. Few psychosocial interventions have been developed for YAs or leverage eHealth modalities to provide supportive care and none have included a focus on enhancing psychological well-being through positive emotions. eHealth interventions represent promising options for patient engagement, especially with ?digital natives? such as YAs. Given the ubiquitous nature of smartphones among YAs and their preference for remotely delivered interventions, the paucity of eHealth interventions among YAs is a missed opportunity. Moreover, although the deleterious effects of psychological distress are well-known, less attention has been focused on the benefits of psychological well-being. Psychological well-being is associated with better health outcomes, unique from the influence of distress, and includes domains inherently valued by young patients. Our team has developed a novel, multicomponent intervention to enhance psychological well-being that shows promise among patients with HIV, diabetes, and breast cancer. We have piloted the intervention for YAs in an eHealth delivery format (EMPOWER: Enhancing Management of Psychological Outcomes With Emotion Regulation) to demonstrate feasibility and acceptability. By leveraging an innovative methodological design, the Multiphase Optimization Strategy (MOST), the main objective of our proposed study is to optimize EMPOWER for YAs and prepare for a future randomized clinical trial (RCT). To accomplish this, we propose the following specific aims: (1) Using a MOST framework, examine which of five components contributes meaningfully to well-being among YA cancer survivors; and (2) Identify mediators and moderators of component efficacy for well-being outcomes. Upon completion of the testing, we will have a fully optimized, eHealth intervention to enhance psychological well-being among YA cancer survivors. This optimized intervention will be primed for a large, multi-site RCT and, as a scalable intervention, it will be ideally-suited for YA survivors who would otherwise not have access to supportive care interventions to help manage post-treatment distress and enhance well-being.